Intracellular Stimulation

Most of the clinically relevant aspects of cardiac stimulation are direct consequences of intracellular properties. Recordings of the electrical responses of single cardiac cells can be obtained with a fine electrical probe inside the cell and by measuring the voltage on that probe relative to the region outside the cell membrane. This probe can be used to manipulate the voltage across the membrane and record the response. The nature of electrical response in single cardiac cells has been characterized by using this technique.

A “resting” cardiac cell maintains a voltage across its membrane, which is called transmembrane potential. This transmembrane voltage (V_m) is normally between 70 and 90 millivolts (mV), with the inside of the cell being negative compared with the outside (extracellular) space. When a cardiac cell is electrically “triggered,” it initiates an action potential. The action potential begins with a very rapid increase in V_m by nearly 100 mV (depolarization). The membrane voltage remains at a slightly positive value for around 200 ms. This is called the plateau phase because the voltage is relatively flat. At the end of the plateau, the cell begins a repolarization phase, returning to the resting voltage in a few tens of milliseconds. The total duration of the action potential can vary significantly, depending on the frequency of cell stimulation, with higher frequencies leading to shorter action potentials. The fundamental characteristic of an action potential is that once it is begun, that is, once it is triggered, it proceeds autonomously.

How is an action potential triggered in a single cell? The cell membrane has a variety of voltage-sensitive ion channels, that is, proteins that allow the passage of a particular species of ion but only at a critical transmembrane voltage (V_t). The action potential is initiated when V_m is reduced to the V_t, at which the sodium (Na⁺) channels are activated, resulting in a large migration of Na⁺ ions into the cell. This is followed by a reversible sequence of changes in the permeability to various other ions, which gives rise to an action potential.

The important parameter, therefore, is the minimum stimulation current required to bring the transmembrane voltage to this critical value (V_t). This minimum current is dependent on two properties of the membrane: capacitance and resistance. If the membrane only had resistance and no capacitance, an increase in stimulus current would change the transmembrane potential as per Ohm’s law (Voltage = Current × Resistance). When enough current (called rheobase current) is injected for the transmembrane potential to reach the critical value, an action potential would be initiated. In such a situation, excitation of the cell would depend only on the current and not on the duration of the stimulation pulse.

The situation gets slightly more complex because of membrane capacitance. Membrane capacitance does not allow the voltage to change instantaneously across the membrane when the stimulus is applied. Charge must be deposited on the membrane to cause a change in V_m. Building this charge takes time, since charge is the product of the stimulation current and its duration. If an applied stimulation pulse fails to excite the cell, it is because the membrane did not reach V_t, so it is necessary to increase the stimulation current, stimulation duration, or both. The relationship between the strength of the stimulus and its duration necessary to initiate excitation is referred to as the strength/duration curve. Irrespective of the duration, the stimulus current must always be greater than the rheobase, since that is the minimum current required to reach V_t across membrane resistance. At shorter durations, more stimulus current is required. Figure 13-1, A, shows the effect of different stimulation currents on the potential V_m of a single cell. Initially, as the current is applied, membrane voltage rises because of membrane capacitance. In the lower trace, V_m reaches a maximum value that is below V_t; then, once the current is discontinued, it goes back to the resting membrane potential (MP). In the upper trace, however, the stimulus current is sufficient to increase membrane voltage above V_t (65 mV in this case), which activates the Na⁺ channels and initiates the action potential. At a constant stimulus amplitude (see Figure 13-1, B), when the stimulus is turned off very quickly after a very short duration, the threshold V_t is not reached because of membrane capacitance, and the cell does not elicit an action potential. When the duration is increased, V_m continues to increase until it reaches the threshold potential V_t, eliciting an action potential. It is this property of cell capacitance that gives rise to a relationship between duration of the stimulus and the amplitude required to elicit excitation (strength-duration curve, as in Figure 13-2). Also, this type of excitation occurs after the stimulus is turned “on” and before it is turned “off” and is referred to as make excitation.

FIGURE 13-1 A, Schematic representation of the effect of increasing amplitude of a constant current stimulus on the transmembrane potential of a cardiac cell. With the smaller amplitude stimulus (red), the membrane potential changes from –85 to about –75 mV and recovers to –85 mV after the stimulus is turned off. With the larger stimulus (blue), the cell transmembrane potential reaches the critical threshold value of –65 mV, and an action potential is initiated. B, The effect of increasing duration of a constant current stimulus on the transmembrane potential of a cardiac cell. With the smaller duration stimulus (red), the membrane potential changes from –85 to about –70 mV and recovers to –85 mV after the stimulus is turned off. When the duration is increased further (blue), the cell transmembrane potential reaches the critical threshold value of –65 mV, and an action potential is initiated.

FIGURE 13-2 Strength/duration curve measured with cathodal and anodal stimuli in a canine ventricle. The pulse durations are plotted on a logarithmic scale. Note that the threshold becomes stable at pulse durations >10 ms; this threshold is called the rheobase. Chronaxie is the stimulus duration required to excite tissue at twice the rheobase.

(From Mehra R: Myocardial vulnerability to arrhythmias with cathodal, anodal and bipolar stimulation [PhD dissertation #75-21384; University Microfilms], Ann Arbor, MI, 1975, University of Michigan.)

It is important to note that the threshold voltage V_t at which excitation occurs is not constant but depends on the extent of cellular repolarization from the previous action potential at the time the stimulus is applied. This is because the Na⁺ channels must reset to their resting state, and this only happens after repolarization is complete. For coupling intervals closer to the repolarization phase (earlier in the cardiac cycle), the threshold voltage V_t increases, thereby increasing the stimulus (either current or duration) needed to initiate excitation. With a depolarizing stimulus (one that reduces V_m), the threshold for excitation is lowest in diastole and increases as the stimulus approaches systole. The threshold voltage V_t (termed critical membrane potential [CMP]; Figure 13-3) and the transmembrane current required to excite cells with intracellular stimulation at various intervals in the cardiac cycle were measured by Hoshi et al in canine Purkinje fibers.¹ Figure 13-3 shows that the CMP was between 60 and 65 mV and relatively constant throughout the diastolic phase and the terminal phase of repolarization of the MP. In the rapid phase of repolarization, the CMP increased sharply and so did the current threshold. This results in a hyperbolic curve that describes the relationship between the current threshold for excitation and the time delay from the onset of the action potential of a depolarizing stimulus. This curve is referred to as the intracellular strength/interval curve and has a shape very similar to the strength/interval curve with an extracellular electrode that results in tissue excitation caused by depolarization (Figure 13-4).

FIGURE 13-3 Critical membrane potential (CMP), depolarizing current threshold (I), and transmembrane potential (V_m) as a function of delay from the onset of an action potential. The ordinate is the potential in millivolts or current threshold and the abscissa is in milliseconds. The CMP is almost constant at intervals longer than 200 ms, and I represents the intracellular strength/interval curve with a depolarizing stimulus.

(From Hoshi TM: Excitability cycle of cardiac muscle examined by intracellular stimulation, Jap J Physiol 12:433, 1962.)

FIGURE 13-4 Typical unipolar cathodal and anodal strength/interval curves obtained in a patient with an acute unipolar electrode (11 mm² in area). The ordinate represents the excitation current threshold in milliamperes, and the abscissa represents the delay of the stimulus from the stimulus pacing the ventricle at 90 beats/min. Note that anodal refractory periods are shorter than cathodal refractory periods for most stimulus currents.

(From Mehra R, Furman S: Comparison of cathodal, anodal and bipolar strength-interval curves with temporary and permanent electrodes, Br Heart J 41:468–476, 1979.)

The second type of excitation occurs when the cardiac cell is hyperpolarized (made more negative than the resting potential) to a critical value after the stimulus is turned off. This is called break excitation. It was initially observed in nerve fibers and associated with the termination of a hyperpolarizing pulse.² Using optical mapping techniques, Wikswo et al³ were also able to demonstrate that the break excitation occurred in cardiac tissue after the end of the stimulus from the region that was hyperpolarized. During the hyperpolarized state, the tissue is inexcitable because its Na⁺ channels are inactivated, and excitation is observed only after the hyperpolarizing stimulus is turned off. There is no consensus on the mechanism responsible for break excitation. Two concepts have been proposed: (1) In the mechanism proposed by Roth et al, break excitation from the hyperpolarized region occurs only when adjacent depolarized areas are present and does not occur in tissue that is uniformly hyperpolarized.⁴ At the end of the stimulus, current diffuses from the depolarized areas to the hyperpolarized areas to initiate excitation. This hypothesis suggests that break excitation is a result of the syncytial properties of the tissue and not a property of the cell membrane alone. (2) Computer simulation conducted by other investigators indicated that break excitation in the hyperpolarized tissue may be possible by itself without the diffusion of current from adjacent tissue.⁵ This controversy has not yet been resolved.

Extracellular Stimulation

To understand extracellular stimulation, it is useful to envision a train of excitation processes, of which intracellular stimulation is the final event. These processes are (1) generation of the stimulus in the pulse generator, (2) transfer of current from the generator to the electrode pair and into the extracellular space, and (3) transfer of current from the extracellular space to the cardiac cell membranes resulting in cellular excitation.

The first two processes will be discussed in this section. The characteristics of the electrical stimulus and the terminology used for extracellular electrodes will be reviewed first. Then we will discuss the stimulus variables that affect extracellular stimulation such as its duration, polarity, and the timing in the cardiac cycle and electrode variables such as the size of electrode and the electrode tissue interface.

Electrical Stimulus

In individual cardiac cells, a minimum threshold current is required to cause excitation. Decades of experimental observations with extracellular electrodes demonstrate the same behavior in cardiac tissue. Stimulus strength may be measured as a voltage or current amplitude. The current threshold is a more physiological measurement of excitation, since it is independent of the resistance of the electrode or the tissue. If the resistance is high, the voltage threshold will increase, as it is equal to the current threshold times the resistance of the tissue. The current density (current per unit area) is an even more accurate determinant of the basic requirement for excitation. It is postulated that in order to elicit excitation, a certain minimum number of cells (a “critical mass”) must be excited. Since the heart is a syncytium, that is, an aggregate of interconnected cardiac cells, once this critical volume of cells is excited, the conduction propagates to the rest of the heart. Therefore, a minimum amount of current per unit area must exist in order to excite this aggregate of cardiac cells. This is called current density threshold. The current density required for cardiac excitation is typically between 2 and 2.5 mA/cm² at the electrode-tissue interface.^6,7

External cardiac stimulators are typically designed to provide an output pulse that maintains its voltage or current throughout the duration of the pulse and are referred to as constant voltage or constant current stimuli. Because of the polarization characteristics of the electrode-tissue interface (to be discussed later), resistance increases during the pulse and an increasing voltage is required to maintain the constant current stimulus. This results in a significant increase from V_o at the beginning of the stimulus to V_t at the end of the stimulus. Figure 13-5, A, illustrates a typical constant current stimulus, where the output current is approximately constant in amplitude from the onset to the offset of the stimulus. Physiological stimulators used in laboratories typically provide constant voltage or constant current stimuli, but these designs draw significant power. To reduce power drain and ensure safe operation in implantable pacemakers, the most efficient circuits use a capacitor. This can continuously draw current from a battery throughout the cardiac cycle and deliver a short stimulus (typically <2 ms) on demand. The result is that the stimulus from most implantable pacemakers has an exponentially decaying voltage profile, that is, a slight decrease occurs in voltage output from the beginning of the stimulus to the end of the stimulus (see Figure 13-5, B). The exponentially decaying voltage output results from a capacitor at the output of the pacemaker. The current waveform that is generated has an even higher rate of decay because of electrode polarization, which results in increasingly higher resistance during the pulse.

FIGURE 13-5 A, Voltage waveform measured at the electrode during a constant current stimulus. The current amplitude is almost constant at 1.0 mA through the stimulus duration of 2 ms. Significantly increasing stimulus voltage from 0.3 V at the beginning of the stimulus to 0.7 V at its end is required to maintain this constant current through the tissue. This is because of the increasing polarization resistance during the pulse. B, Voltage (top) and current (bottom) waveform of a 4-V, 0.4-ms stimulus measured at the output of a typical pacemaker. The voltage is decaying exponentially during the pulse and is followed by a “recharge” of an opposite polarity. The current waveform decays at a greater rate because of increasing resistance from the beginning of the stimulus to the end. After the end of the 0.4-ms stimulus, the current returns to zero and is followed by a recharge stimulus of a small magnitude but longer duration.

(A, From Furman S, Parker B, Escher DJW, et al: Endocardial threshold of cardiac response as a function of electrode surface area, J Surg Res 8:161–166, 1968.)

Electrode Terminology

Electrical stimulation must always be applied through electrode pairs so that current injected at one electrode returns to the stimulator through the other electrode. Figure 13-6 shows the commonly used electrode configurations. When one of the electrodes is located inside the heart and another is located away from the heart (e.g., on the skin surface or subcutaneously), this configuration is described as unipolar. The electrode in contact with (or closest to) the heart is called the stimulating electrode, and the electrode away from the heart is called the indifferent electrode. The indifferent electrode is usually quite large compared with the stimulating electrode to lower the resistance of the stimulation configuration. When the stimulating electrode is negative with respect to the indifferent electrode, the stimulation is called cathodal. When the stimulating electrode is positive, the stimulation is referred to as anodal.

FIGURE 13-6 Unipolar and bipolar electrode configurations. The normal unipolar pacing configuration is one in which the tip of the electrode is negative (distal cathodal) with an indifferent positive electrode. The unipolar distal anodal configuration reverses the polarities and is not used because of higher anodal thresholds. The normal bipolar configuration uses the tip (distal) electrode as the cathode and the ring (proximal) electrode as the anode.

(From Mehra R, Furman S: Comparison of cathodal, anodal and bipolar strength-interval curves with temporary and permanent electrodes, Br Heart J 41:468–476, 1979.)

In most implanted systems, both electrodes are within the same chamber of the heart, usually separated by approximately 1 cm. This configuration is described as bipolar. In practice, the distal of the two electrodes, that is, “the tip,” is more likely to be in contact with tissue than is the more proximal electrode (see Figure 13-6).